The present invention relates to the art of earth boring and more particularly to a raise bit for enlarging a pilot hole into a larger diameter hole by disintegrating the earth formations surrounding the pilot hole.
A relatively large diameter hole may be provided between a first location and a second location by an operation commonly referred to as raise drilling. The raise drilling operation begins by drilling a small diameter pilot hole through the earth from a first location to an opening at a second location using a small diameter pilot bit. After the pilot hole is completed, the pilot bit is removed from the drill column and a larger diameter raise bit attached. The raise bit is rotated and drawn along the pilot hole thereby enlarging the pilot hole to the desired size.
On many occasions, the small diameter pilot hole extends to an area accessible only through a small drift or passage. The removable drive stem of the present invention provides the raise bit with a low profile thereby allowing the raise bit to be transported through low drifts or passages. For example, a 72-inch diameter raise bit may be transported through a drift having a 30-inch height whereas a 72-inch diameter raise bit without the removable stem requires a drift with at least a 52-inch height.
During the raise drilling operation, a tremendous amount of wear and stress is imposed upon the raise bit. Generally, the pilot hole tends to wander which results in a series of curves over the length of the pilot hole. As the raise bit is drawn along the pilot hole, the drilling geometry changes. This results in a tremendous amount of stress being transmitted to the raise bit when it is being drawn through the curves. The outside radius of the raise bit may be many times greater than the radius of the pilot hole; therefore, the moment developed is great. When resistance is encountered by cutters located on the outer radius of the raise bit as when drilling along a curve, it causes increased stress on the raise bit. It can be appreciated that changes in formations encountered during the raise drilling operation complicates the above-mentioned conditions and adds to the drilling difficulties.
The tremendous stress and wear encountered during the raise drilling operation causes some elements of the raise bit to wear out much earlier than others, notably the cutters and the drive stem. The bit of the present invention allows the elements that have a relatively short lifetime to be replaced thereby extending the lifetime of the bit and reducing cost. The drive stem is removable and, therefore, replaceable as are the individual cutters. This allows the elements that receive the greatest stress and wear to be readily replaced. The relatively short lifetime of the individual cutters results from their nearly constant contact with the formations and the consequent exposure to wear and abuse. The drive stem also contacts the formations and receives a great deal of wear and abuse. The drive stem absorbs a large amount of stress since it is the single link between the drill column and the body of the bit. All energy from the rotary equipment is transmitted to the bit through the drive stem.
The drive stem is often subject to failure prior to failure of the body of the raise bit. It is, therefore, desirable to be able to easily and quickly remove the stem from the raise bit and replace it with a new stem. The stem is generally constructed of a high-strength, heat-treated material and would be weakened by welding. It is, therefore, desirable to be able to attach and remove the stem from the raise bit body without welding. From the foregoing, it will be appreciated that the stem must be constructed of material that will stand up under the harsh conditions encountered. Such material is expensive and any reduction in the amount of this material used is cost savings. The bit of the present invention allows the stem to be manufactured from high-strength material, whereas the body may include lower strength and, therefore, less expensive materials.
The stem or a stabilizer section on the stem should be nearly the size of the pilot hole to insure a smooth raise drilling operation. In order to use the same bit body in pilot holes of different sizes, it is desirable to be able to remove the stem from the raise bit body and replace it with an appropriately sized stem. The stem should be easy to remove and replace using simple hand tools. This allows the stem to be replaced in the field under adverse conditions.